Synthetic crystalline silicate materials are now used extensively as catalysts in a number of industries, especially the petroleum refining, petrochemical and chemical industries. These synthetic silicate catalytic materials are generally characterized as being solid, porous, crystalline silica-containing materials whose utility as catalysts is predicated upon their possession of defined and characteristic pore sizes and ordered, internal structures which confer specific catalytic properties on these materials. The most common class of synthetic silicate catalysts has been the aluminosilicate zeolites and of these, the materials which have probably been produced in the largest quantities are the large pore size aluminosilicate zeolites, exemplified by the synthetic faujasites zeolite X and zeolite Y, which are widely used in the catalyst cracking process of converting high-boiling petroleum feeds to lower-boiling products, especially gasoline, as well as in the hydrocracking process which also forms an important part of petroleum refinery operations. The other class of synthetic silicate catalytic materials which are produced in large quantities are the intermediate pore size silicates, especially the intermediate pore size aluminosilicate zeolites, such as ZSM-5, which are important catalysts in many petrochemical processes such as the isomerization of xylenes, the disproportionation of toluene, the production of various para-substituted aromatic compounds, e.g. paraethyltoluene, as well as in petroleum refining processes, especially catalytic dewaxing, e.g. the dewaxing of distillates and the dewaxing of lubricants. The intermediate pores size materials which have achieved the greatest success are the aluminosilicate zeolites and these may have various silica:alumina ratios, typically from about 30:1 or higher, e.g. 70:1, 200:1 or ever higher. It is, however, not required that the silicate should be an aluminosilicate because other trivalent metals may provide the required acidic functionality which characterizes these catalytic materials. For example, U.S. Pat. No. 3,702,886, which describes ZSM-5 discloses the possibility for using gallium as a substituted for aluminum, U.S. Pat. No. 4,269,813 and BE 859,056 discloses borosilicate materials, and U.S. Pat. No. 4,238,318 discloses ferrosilicates which also possess shape-selective catalytic properties characterized by the possession of acidic functionality at internal sites within the structure of the silicate to which access is controlled by the characteristic crystalline structure of the silicate. Other trivalent cations such as chromium or phosphorus may also be present in these silicates, as disclosed, for example, in U.S. Pat. Nos. 4,414,423; 4,417,086; 4,517,396 and 4,309,280. The presence of metals within the silicate structure is, in fact, by no means essential to the crystalline structure or to the possession of catalytic properties, as disclosed in U.S. Pat. No. 3,941,871. A material described as a silica polymorph is described in U.S. Pat. No. 4,061,724 which has now been established to be zeolite ZSM-5, Nature 296, 530 (1982), J. Catalysis 61, 390-396 (1980). An organosilicate with very high silica content is described in U.S. Pat. No. 3,941,871. Thus, notwithstanding differences in the specific compositions of these various silicate materials, they are considered to have a sufficient community that they are regarded as belonging to a defined class with recognized common characteristics.
A number of synthetic intermediate pore size zeolites are now known which are useful for their shape selective catalytic properties. Among them are zeolites ZSM-5, ZSM-11, ZSM-22, theta-1 which is isostructural with ZSM-22 and ZSM-23. These zeolites, their properties and utilites are described in Catal. Rev.-Sci. Eng. 28 92&3), 185-264 (1986).
Other synthetic zeolites include, for example, zeolite beta (U.S. Pat. No. 3,308,069), synthetic mordenites including TEA mordentite, TMA-offretite and large pore size zeolites including ZSM-20 and ZSM-4. These materials have been investigated for various utilites in the petrochemical and petroleum refining industries and many uses for them have been found.
These silicate materials are conventionally produced by the crystallization of the silicate from an aqueous gel or slurry which is being prepared by adding a source of silica together with other appropriate ingredients to water and permitting the crystallization to occur under defined conditions which promote the crystallization of the desired species. Silica may be provided by various sources including silica itself in the form of colloidal silica, precipitated silica, silica gel, silica hydrosols or of silica compounds including silicic acid, metal silicates especially sodium silicate or other alkali metal silicates and metallosilicates including aluminosilicates, e.g. sodium aluminosilicates, and other materials which will function as a source of silica for the zeolite. The silica source may also function as a source of other components of the zeolite, for example, sodium aluminosilicate also functions as a source of aluminum. When aluminosilicate are being produced, the aqueous synthesis mixture usually contains a source of silica, a source of alumina, such as an aluminum salt, e.g. aluminum sulfate or aluminum nitrate, water and, in many cases, an organic directing agent of "template" which promotes the formation of the desired species, for example, an amine or a tetra alkylammonium cation such as tetrapropylammonium (TPA) or tetraethylammonium (TEA) cations. U.S. Pat. No. 3,702,886, for example, discloses the use of tetraalkylammonium cations, especially TPA, for the preparation of ZSM-5; U.S. Pat. No. 4,139,600 discloses the use of alkyldiamines; U.S. Pat. No. 4,296,083 discloses the use of ethylenediamine and other amines including trialkylamines, U.S. Pat. No. 4,151,189 discloses the use of various primary amines as a directing agent for ZSM-5, ZSM-12, ZSM-35 and ZSM-38; U.S. Pat. No. 4,565,681 discloses the use of mixed-alkyl ammonium compounds; and U.S. Pat. No. 4,100,262 discloses the use of tetraalkylammonium compounds in combination with a tetraurea cobalt (II) complex. Other systems are also known. However, the presence of the organic component is not necessary since it is possible to produce selected aluminosilicate zeolite species without the use of a directing agent under particular, defined conditions, as described in U.S. Pat. Nos. 4,175,114; 4,119,556; 4,157,885 and 4,341,748 to which reference is made for a description of such processes. Furthermore, control of the composition of the synthesis mixture may result in different zeolites being produced; to take one instance, zeolite beta may be produced using a TEA component under defined ranges of mixture composition, whereas mordenite may be produced under other defined ranges. The effect of these compositional changes is, however, established and there is a significant predictability in the species which are produced from any particular synthesis mixture. This has resulted in the large scale commercial preparation of aluminosilicate zeolites such as the synthesis faujasites especially zeolite Y, zeolite beta and in various forms of ZSM-5 with differing silica: alumina ratios, as well as in the production of certain borosilicate catalysts.
The manufacturing processes used in the commercial scale synthesis of silicate catalytic materials of the types described above conventionally employ large vats or autoclaves for step-wise mixing, gel aging and final crystallization of the product. Processes of this type are reviewed in "Zeolite Molecular Sieves", D.W. Breck, John Wiley and Sons, New York, 1974, Ch. 9 and "Zeolite Chemistry and Catalysts", J. A. Rabo, American Chemical Society, Washington, D.C., 1976. In general, the zeolites have been produced in batch type processes, using large autoclaves, either static or stirred. They may, however, be produced in a continuous process as referred to, for example, in Belgium Pat. No. 869,156, to which reference is made for details of such a continuous process.
Regardless of whether the process is batch or continuous in nature, it is subject to the kinetic limitations of the crystallization mechanism and, generally, this requires a significant amount of time. In order to reduce the expense of the process, it is customary to monitor the degree of crystallinity which has been achieved so that crystallization may be terminated as soon as the product achieves a requisite minimum crystallinity. In the past, this termination of crystallinity has been made by withdrawing a sample of the synthesis mixture and measuring its crystallinity by means of X-ray diffraction of a dried sample. This is relatively intensive in terms of its requirements in time and labor and is generally not suitable for monitoring the progress of crystallization in a continuous operation since it does not provide results rapidly enough to permit satisfactory control of the process variables. It would therefore be desirable to have a more rapid method of determining the progress of the crystallization both for the batch and continuous procedures.
Although measurement of the pH of the crystallization mixture might suggest itself as a means for determining the progress of the crystallization, it is subject to some objections. First, the glass electrodes used for its measurement are fragile which generally precludes their use in the synthesiser vessel itself so that, again, sample withdrawal is required and this debars the technique for use in a continuous synthesis where real-time monitoring of the crystallization is required.